Shown in FIG. 1 is a simplified schematic arrangement of a gas turbine 10 which is operated with a gaseous fuel 16. The gas turbine 10 comprises a compressor 11 which draws in and compresses fresh air 14 and feeds the compressed air 15 to a combustion chamber 12 where it is used for combusting the gaseous fuel 16. The resulting hot gas 17 is expanded in a subsequent turbine 13, performing work, and is discharged as exhaust gas 18. The turbine 13 drives the compressor 11 and a generator 21 for generating electric power. The feed of the gaseous fuel 16 to the combustion chamber 12 is controlled and regulated by a control valve 19 which is connected to a control unit 20. An example of such controlling is described in EP 1 310 647.
Problems during operation of a gas turbine include vibrations in the control system of the gas turbine. There are various causes of such vibrations in the gas turbine control system. One of them is primarily caused by an excessively low pressure drop at the control valve 19 for the fuel gas feed.
A widely used means against a low gas pressure is the so-called variable-pressure operation in which the load of the gas turbine is reduced in order to lower the necessary gas pressure, specifically based either on (a) a fixed maximum control valve stroke limit value, or on (b) the compressor output pressure. Such a procedure is described in U.S. Pat. No. 7,549,293 B2, for example.
In order to maintain the necessary fuel mass flow, the control valve stroke is automatically increased as gas supply becomes lower. For this reason, monitoring of the control valve stroke (variant a) is a more accurate method for detecting a low gas pressure in comparison to monitoring of the compressor output pressure (variant b). The disadvantage of variant (a) is that a fixed maximum control valve stroke limit value has to be determined individually on the basis of the dimensions of the control valve.
The control valve stroke application is customarily designed so that all contractually agreed boundary conditions for the ambient temperature and the fuel gas composition are satisfied. However, it can be that the valve is oversized for certain boundary conditions. Such an oversized control valve can have a low pressure drop in such a way that the gas turbine control system becomes unstable.
Monitoring of the compressor output pressure according to variant (b) means a protection against an excessively low fuel gas supply pressure. The disadvantage of this variant, however, is that the protection is based on fixed variables which are calculated from accepted boundary conditions. These conditions must also cover the “worst case”, which leads to a comparatively conservative protection and results in an avoidable reduction of the reliability of the machine.
If, therefore, variant (b) is replaced by variant (a), it must be ensured that the gas turbine can be operated in a stable manner, specifically independently of the dimensions of the control valve and independently of the ambient conditions.